RAD6 inhibition enhances paclitaxel sensitivity of triple negative breast cancer cells by aggravating mitotic spindle damage

Background

Paclitaxel (PTX), a first-line therapy for triple negative breast cancers (TNBC) induces anti-tumor activity by microtubule stabilization and inhibition of cell division. Its dose-limiting toxicity and short half-life, however, pose clinical challenges underscoring the need for strategies that increase its efficiency. RAD6, a E2 ubiquitin conjugating enzyme, is associated with centrosomes at all phases of cell cycle. Constitutive overexpression of the RAD6B homolog in normal breast cells induces centrosome amplification and multipolar spindle formation, indicating its importance in centrosome regulation.

Methods

TNBC centrosome numbers were scored by pericentrin immunostaining. PTX sensitivities and interactions with SMI#9, a RAD6-selective small molecule inhibitor, on TNBC cell survival were analyzed by MTT and colony forming assays and an isogenic MDA-MB-468 TNBC model of PTX resistance. The molecular mechanisms underlying PTX and SMI#9 induced cytotoxicity were determined by flow cytometry, immunoblot analysis of cyclin B1 and microtubule associated protein TAU, and dual immunofluorescence staining of TAU and α-tubulin.

Results

Our data show aberrant centrosome numbers and that PTX sensitivities are not correlated with TNBC BRCA1 status. Combining PTX with SMI#9 synergistically enhances PTX sensitivities of BRCA1 wild-type and mutant TNBC cells. Whereas SMI#9/PTX combination treatment increased cyclin B1 levels in MDA-MB-468 cells, it induced cyclin B1 loss in HCC1937 cells with accumulation of reproductively dead giant cells, a characteristic of mitotic catastrophe. Cell cycle analysis revealed drug-induced accumulation of tetraploid cells in S and G2/M phases, and robust increases in cells with 4 N DNA content in HCC1937 cells. TAU overexpression is associated with reduced PTX efficacy. Among the six TAU isoforms, both SMI#9 and PTX downregulated 1N3R TAU in MDA-MB-468 and HCC1937 cells, suggesting a common mechanism of 1N3R regulation. Dual TAU and α-tubulin immunostaining showed that SMI#9 induces monopolar mitotic spindles. Using the isogenic model of PTX resistance, we show that SMI#9 treatment restores PTX sensitivity.

Conclusions

These data support a common mechanism of microtubule regulation by SMI#9 and PTX and suggest that combining PTX with RAD6 inhibitor may be beneficial for increasing TNBC sensitivities to PTX and alleviating toxicity. This study demonstrates a new role for RAD6 in regulating microtubule dynamics.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-10119-z.

RAD6B Loss Disrupts Expression of Melanoma Phenotype in Part by Inhibiting WNT/β-Catenin Signaling

Canonical Wnt signaling is critical for melanocyte lineage commitment and melanoma development. RAD6B, a ubiquitin-conjugating enzyme critical for translesion DNA synthesis, potentiates β-catenin stability/activity by inducing proteasome-insensitive polyubiquitination. RAD6B expression is induced by β-catenin, triggering a positive feedback loop between the two proteins. RAD6B function in melanoma development/progression was investigated by targeting RAD6B using CrispR/Cas9 or an RAD6-selective small-molecule inhibitor #9 (SMI#9). SMI#9 treatment inhibited melanoma cell proliferation but not normal melanocytes. RAD6B knockout or inhibition in metastatic melanoma cells downregulated β-catenin, β-catenin-regulated microphthalmia-associated transcription factor (MITF), sex-determining region Y-box 10, vimentin proteins, and MITF-regulated melan A. RAD6B knockout or inhibition decreased migration/invasion, tumor growth, and lung metastasis. RNA-sequencing and stem cell pathway real-time RT-PCR analysis revealed profound reductions in WNT1 expressions in RAD6B knockout M14 cells compared with control. Expression levels of β-catenin-regulated genes VIM, MITF-M, melan A, and TYRP1 (a tyrosinase family member critical for melanin biosynthesis) were reduced in RAD6B knockout cells. Pathway analysis identified gene networks regulating stem cell pluripotency, Wnt signaling, melanocyte development, pigmentation signaling, and protein ubiquitination, besides DNA damage response signaling, as being impacted by RAD6B gene disruption. These data reveal an important and early role for RAD6B in melanoma development besides its bonafide translesion DNA synthesis function, and suggest that targeting RAD6B may provide a novel strategy to treat melanomas with dysregulated canonical Wnt signaling.

Alternative Splicing of RAD6B and Not RAD6A is Selectively Increased in Melanoma: Identification and Functional Characterization

Rad6B, a principal component of the translesion synthesis pathway, and activator of canonical Wnt signaling, plays an essential role in cutaneous melanoma development and progression. As Rad6 is encoded by two genes, namely, UBE2A (RAD6A) and UBE2B (RAD6B), in humans, we compared their expressions in melanomas and normal melanocytes. While both genes are weakly expressed in normal melanocytes, Rad6B is more robustly expressed in melanoma lines and patient-derived metastatic melanomas than RAD6A. The characterization of RAD6B transcripts revealed coexpression of various splice variants representing truncated or modified functional versions of wild-type RAD6B in melanomas, but not in normal melanocytes. Notably, two RAD6B isoforms with intact catalytic domains, RAD6BΔexon4 and RAD6Bintron5ins, were identified. We confirmed that RAD6BΔexon4 and RAD6Bintron5ins variants are expressed as 14 and 15 kDa proteins, respectively, with functional in vivo ubiquitin conjugating activity. Whole exome sequence analysis of 30 patient-derived melanomas showed RAD6B variants coexpressed with wild-type RAD6B in all samples analyzed, and RAD6Bintron5ins variants were found in half the cases. These variants constitute the majority of the RAD6B transcriptome in contrast to RAD6A, which was predominantly wild-type. The expression of functional RAD6B variants only in melanomas reveals RAD6B's molecular heterogeneity and its association with melanoma pathogenesis.

Crosstalk between translesion synthesis, Fanconi anemia network, and homologous recombination repair pathways in interstrand DNA crosslink repair and development of chemoresistance

Bifunctional alkylating and platinum based drugs are chemotherapeutic agents used to treat cancer. These agents induce DNA adducts via formation of intrastrand or interstrand (ICL) DNA crosslinks, and DNA lesions of the ICL type are particularly toxic as they block DNA replication and/or DNA transcription. However, the therapeutic efficacies of these drugs are frequently limited due to the cancer cell's enhanced ability to repair and tolerate these toxic DNA lesions. This ability to tolerate and survive the DNA damage is accomplished by a set of specialized low fidelity DNA polymerases called translesion synthesis (TLS) polymerases since high fidelity DNA polymerases are unable to replicate the damaged DNA template. TLS is a crucial initial step in ICL repair as it synthesizes DNA across the lesion thus preparing the damaged DNA template for repair by the homologous recombination (HR) pathway and Fanconi anemia (FA) network, processes critical for ICL repair. Here we review the molecular features and functional roles of TLS polymerases, discuss the collaborative interactions and cross-regulation of the TLS DNA damage tolerance pathway, the FA network and the BRCA-dependent HRR pathway, and the impact of TLS hyperactivation on development of chemoresistance. Finally, since TLS hyperactivation results from overexpression of Rad6/Rad18 ubiquitinating enzymes (fundamental components of the TLS pathway), increased PCNA ubiquitination, and/or increased recruitment of TLS polymerases, the potential benefits of selectively targeting critical components of the TLS pathway for enhancing anti-cancer therapeutic efficacy and curtailing chemotherapy-induced mutagenesis are also discussed.

Rad6 is a Potential Early Marker of Melanoma Development

Melanoma is the leading cause of death from skin cancer in industrialized countries. Several melanoma-related biomarkers and signaling pathways have been identified; however, their relevance to melanoma development/progression or to clinical outcome remains to be established. Aberrant activation of Wnt/β-catenin pathway is implicated in various cancers including melanoma. We have previously demonstrated Rad6, an ubiquitin-conjugating enzyme, as an important mediator of β-catenin stability in breast cancer cells. Similar to breast cancer, β-catenin-activating mutations are rare in melanomas, and since β-catenin signaling is implicated in melanoma, we examined the relationship between β-catenin levels/activity and expression of β-catenin transcriptional targets Rad6 and microphthalmia-associated transcription factor-M (Mitf-M) in melanoma cell models, and expression of Rad6, β-catenin, and Melan-A in nevi and cutaneous melanoma tissue specimens. Our data show that Rad6 is only weakly expressed in normal human melanocytes but is overexpressed in melanoma lines. Unlike Mitf-M, Rad6 overexpression in melanoma lines is positively associated with high molecular weight β-catenin protein levels and β-catenin transcriptional activity. Double-immunofluorescence staining of Rad6 and Melan-A in melanoma tissue microarray showed that histological diagnosis of melanoma is significantly associated with Rad6/Melan-A dual positivity in the melanoma group compared to the nevi group (P=.0029). In contrast to strong β-catenin expression in normal and tumor areas of superficial spreading malignant melanoma (SSMM), Rad6 expression is undetectable in normal areas and Rad6 expression increases coincide with increased Melan-A in the transformed regions of SSMM. These data suggest a role for Rad6 in melanoma pathogenesis and that Rad6 expression status may serve as an early marker for melanoma development.

Comedo-DCIS is a precursor lesion for basal-like breast carcinoma: identification of a novel p63/Her2/neu expressing subgroup

Basal breast cancer comprises ~15% of invasive ductal breast cancers, and presents as high-grade lesions with aggressive clinical behavior. Basal breast carcinomas express p63 and cytokeratin 5 (CK5) antigens characteristic of the myoepithelial lineage, and typically lack Her2/neu and hormone receptor expression. However, there is limited data about the precursor lesions from which they emerge. Here we wished to determine whether comedo-ductal carcinoma in situ (comedo- DCIS), a high-risk in situ breast lesion, serve as precursors for basal-like breast cancer. To determine this link, p63, CK5, Her2/neu, epidermal growth factor receptor (EGFR), estrogen receptor (ER) and progesterone receptor (PgR) expression were analyzed by immunohistochemistry in 17 clinical comedo- and 12 noncomedo-DCIS cases, and in tumors derived from unfractionated and CK5-overexpressing subpopulation (MCF10DCIS.com-CK5(high)) of MCF10DCIS.com cells, a model representative of clinical comedo-DCIS. p63 and Her2/neu coexpression was analyzed by immunofluorescence double labeling. A novel p63/CK5/Her2/neu expressing subpopulation of cells that are ER-/PgR-/EGFR- were identified in the myoepithelial and luminal areas of clinical comedo-DCIS and tumors derived from unfractionated MCF10DCIS.com and MCF10DCIS.com-CK5(high) cells. These data suggest that p63 and Her2/neu expressors may share a common precursor intermediate. P63, but not Her2/neu, expression was significantly associated (P = 0.038) with microinvasion/recurrence of clinical comedo-DCIS, and simultaneous expression of p63 and Her2/neu was marginally associated (P = 0.067) with comedo-DCIS. These data suggest that p63/Her2/neu expressing precursor intermediate in comedo-DCIS may provide a cellular basis for emergence of p63+/Her2/neu- or p63+/Her2/neu+ basal-like breast cancer, and that p63/Her2/neu coexpression may serve as biomarkers for identification of this subgroup of basal-like breast cancers.

Ocimum gratissimum retards breast cancer growth and progression and is a natural inhibitor of matrix metalloproteases

Ocimum genus (a.k.a holy basil or tulsi) is a dietary herb used for its multiple beneficial pharmacologic properties including anti-cancer activity. Here we show that crude extract of Ocimum gratissimum (OG) and its hydrophobic and hydrophilic fractions (HB and HL) differentially inhibit breast cancer cell chemotaxis and chemoinvasion in vitro and retard tumor growth and temporal progression of MCF10ADCIS.com xenografts, a model of human breast comedo-ductal carcinoma in situ (comedo-DCIS). OG-induced inhibition of tumor growth was associated with decreases in basement membrane disintegration, angiogenesis and MMP-2 and MMP-9 activities as confirmed by in situ gelatin zymography and cleavage of galectin-3. There was also decrease in MMP-2 and MMP-9 activities in the conditioned media of OG-treated MCF10AT1 and MCF10AT1-EIII8 premalignant human breast cancer cells as compared with control. The MMP-2 and MMP-9 inhibitory activities of OG were verified in vitro using gelatin, a synthetic fluorogenic peptide and recombinant galectin-3 as MMP substrates. Mice fed on OG-supplemented drinking water showed no adverse effects compared with control. These data suggest that OG is non-toxic and that the anti-cancer therapeutic activity of OG may in part be contributed by its MMP inhibitory activity.

Novel inhibitors of Rad6 ubiquitin conjugating enzyme: design, synthesis, identification, and functional characterization

Protein ubiquitination is important for cell signaling, DNA repair, and proteasomal degradation, and it is not surprising that alterations in ubiquitination occur frequently in cancer. Ubiquitin-conjugating enzymes (E2) mediate ubiquitination by selective interactions with ubiquitin-activating (E1) and ubiquitin ligase (E3) enzymes, and thus selective E2 small molecule inhibitor (SMI) will provide specificity unattainable with proteasome inhibitors. Here we describe synthesis and functional characterization of the first SMIs of human E2 Rad6B, a fundamental component of translesion synthesis DNA repair. A pharmacophore model for consensus E2 ubiquitin-binding sites was generated for virtual screening to identify E2 inhibitor candidates. Twelve triazine (TZ) analogs screened in silico by molecular docking to the Rad6B X-ray structure were verified by their effect on Rad6B ubiquitination of histone H2A. TZs #8 and 9 docked to the Rad6B catalytic site with highest complementarity. TZs #1, 2, 8, and 9 inhibited Rad6B-ubiquitin thioester formation and subsequent ubiquitin transfer to histone H2A. SMI #9 inhibition of Rad6 was selective as BCA2 ubiquitination by E2 UbcH5 was unaffected by SMI #9. SMI #9 more potently inhibited proliferation, colony formation, and migration than SMI #8, and induced MDA-MB-231 breast cancer cell G2-M arrest and apoptosis. Ubiquitination assays using Rad6 immunoprecipitated from SMI #8- or 9-treated cells confirmed inhibition of endogenous Rad6 activity. Consistent with our previous data showing Rad6B-mediated polyubiquitination stabilizes β-catenin, MDA-MB-231 treatment with SMIs #8 or 9 decreased β-catenin protein levels. Together these results describe identification of the first Rad6 SMIs.

Lysine 394 is a novel Rad6B-induced ubiquitination site on beta-catenin

The ubiquitin conjugating enzyme Rad6B is overexpressed in breast cancer and induces β-catenin transcriptional activation and stabilization via K63-linked polyubiquitination. Here we identify β-catenin and Rad6B interacting regions, identify potential Rad6B ubiquitination sites in β-catenin, and characterize their breast cancer tissue expression. β-catenin and Rad6B colocalize in breast carcinoma and coimmunoprecipitate from MDA-MB-231 cells. Pull-down assays using GST-β-catenin and His-Rad6B deletion mutants identified amino acids 131-181 and 50-116, respectively, as necessary for their interaction. Ubiquitination assays using β-catenin deletion mutants mapped Rad6B-induced ubiquitination within β-catenin 181-422 encompassing Armadillo repeats 2-7. Lysine to arginine mutations within repeats 5-7 identified K394 as the major Rad6B ubiquitination site in vitro and in vivo, and confirmed by Rad6B ubiquitination of a β-catenin peptide encompassing K394. Ubiquitination of wild type- but not K394R-β-catenin was decreased by Rad6B silencing. Compared to wild type-, K312R-, K335R-, K345R-, or K354R-β-catenin, K394R mutation caused ~50% drop in TOP/Flash activity in Wnt-silent MCF-7 cells. Consistent with these data, expression of Rad6B, itself a β-catenin/TCF transcriptional target, was also reduced in K394R-β-catenin transfected cells. Steady-state K394R-β-catenin levels are decreased compared to wild type-β-catenin. The decreased expression is not due to proteasomal degradation as treatment with MG132 failed to rescue its levels. Lymph node-positive breast carcinomas express higher levels of Rad6 protein and Rad6 activity, and K63-linked ubiquitinated β-catenin than reduction mammoplasties. These data suggest that K394 is a novel site of β-catenin ubiquitination that may be important for the stability and activity of β-catenin in breast cancer.

Drug resistance: challenges to effective therapy

The success of current treatment strategies is limited by the development of therapy resistance as evidenced by recurrence of the primary tumor or distant metastasis. Eradication of primary and metastatic disease requires interventions at both the cancer cell and tumor microenvironment levels. In this review, we will discuss mechanisms that are intrinsic to cancer cells, and those that are mediated by the tumor microenvironment as contributors to drug resistance. Mechanisms contributing to multidrug resistance phenotype and the challenges facing molecular targeted therapy are discussed. The DNA damage tolerance pathway confers tolerance to a variety of structurally and functionally unrelated drugs. A rationale for targeting the DNA damage tolerance pathway as a novel tool for overcoming drug resistance is discussed. We have also addressed the need for employing clinically relevant model systems for performing drug sensitivity evaluations. These model systems must take into account the three-dimensional organization and in vivo relationship of tumor with its microenvironment. Such integrative efforts would not only yield a more global understanding of the tumor- and microenvironment-derived mechanisms involved in emergence of drug resistance but would also provide novel therapeutic targets that will disrupt the interactions between the tumor cells and its microenvironment.

Nanoparticle-mediated combination chemotherapy and photodynamic therapy overcomes tumor drug resistance

Tumor drug resistance significantly limits the success of chemotherapy in the clinic. Tumor cells utilize multiple mechanisms to prevent the accumulation of anticancer drugs at their intracellular site of action. In this study, we investigated the anticancer efficacy of doxorubicin in combination with photodynamic therapy using methylene blue in a drug-resistant mouse tumor model. Surfactant-polymer hybrid nanoparticles formulated using an anionic surfactant, Aerosol-OT (AOT), and a naturally occurring polysaccharide polymer, sodium alginate, were used for synchronized delivery of the two drugs. Balb/c mice bearing syngeneic JC tumors (mammary adenocarcinoma) were used as a drug-resistant tumor model. Nanoparticle-mediated combination therapy significantly inhibited tumor growth and improved animal survival. Nanoparticle-mediated combination treatment resulted in enhanced tumor accumulation of both doxorubicin and methylene blue, significant inhibition of tumor cell proliferation, and increased induction of apoptosis. These data suggest that nanoparticle-mediated combination chemotherapy and photodynamic therapy using doxorubicin and methylene blue has significant therapeutic potential against drug-resistant tumors.

Utility of DNA postreplication repair protein Rad6B in neoadjuvant chemotherapy response

Neoadjuvant chemotherapy is a standard therapy for patients with locally advanced breast cancer (LABC) and is increasingly used for early stage operable breast cancer. Not all patients benefit from it, and reliable markers for predicting response are needed. The cytotoxic effects of chemotherapy are mediated by induction of DNA damage in tumor cells. There is evidence that resistance to chemotherapy is related to enhanced repair of DNA lesions. The postreplication DNA repair (PRR) or translesion synthesis backup DNA repair pathway is critical for cell viability, conferring tolerance to DNA damaging drugs, and maintenance of genomic integrity. However, despite its importance in conferring tolerance to a variety of DNA damaging drugs including cytotoxic chemotherapy, the involvement of this backup repair pathway in chemotherapy response has not been studied. The Rad6B protein is a fundamental component of PRR. We have shown previously that the ability of breast cells to tolerate chemotherapeutic drugs correlates with Rad6B expression levels and PRR capacity. To determine whether Rad6B expression/distribution can be used singly or in combination with p53, Mdr-1/PgP, PCNA or beta-catenin as predictors of response to neoadjuvant chemotherapy, we analyzed posttreatment samples from 20 patients with LABC in a retrospective study. Only preferential Rad6B nuclear localization was associated with response to neoadjuvant chemotherapy. Nuclear exclusion with cytoplasmic overexpression of Rad6B was observed in some patients who failed to respond, but the association with response is not statistically significant. This is the first study to report that the postreplication DNA repair protein Rad6B could be used as an independent marker for determining response to neoadjuvant chemotherapy. This is an exploratory study and larger studies utilizing interim evaluations of Rad6B expression, its subcellular localization and repair activity are required to confirm its utility as a predictor of chemotherapeutic response.

Comedo-ductal carcinoma in situ: A paradoxical role for programmed cell death

Comedo-DCIS is a histologic subtype of preinvasive breast neoplasia that is characterized by prominent apoptotic cell death and has greater malignant potential than other DCIS subtypes. We investigated the mechanisms of apoptosis in comedo-DCIS and its role in conversion of comedo-DCIS to invasive cancer. Clinical comedo-DCIS excisions and the MCF10DCIS.com human breast cancer model which produces lesions resembling comedo-DCIS were analyzed. Apoptotic luminal and myoepithelial cells were identified by TUNEL and reactivity to cleaved PARP antibody and cell death assessed by Western blotting, Mitocapture and immunohistochemical assays. MCF10DCIS.com cells undergo spontaneous apoptosis in vitro, both in monolayers and multicellular spheroids; it is associated with increased mitochondrial membrane permeability, increase in Bax/Bcl-2 ratio and occurs via caspase-9-dependent p53-independent pathway. This suggests that apoptosis is stromal-independent and that the cells are programmed to undergo apoptosis. Immunostaining with cleaved PARP antibody showed that myoepithelial apoptosis occurs before lesions progress to comedo-DCIS in both clinical comedo-DCIS and in vivo MCF10DCIS.com lesions. Intense staining for MMP-2, MMP-3, MMP-9 and MMP-11 was observed in the stroma and epithelia of solid DCIS lesions prior to conversion to comedo-DCIS in clinical and MCF10DCIS.com lesions. Gelatin zymography showed higher MMP-2 levels in lysates and conditioned media of MCF10DCIS. com cells undergoing apoptosis. These data suggest that signals arising from the outside (microenvironmental) and inside (internal genetic alterations) of the duct act in concert to trigger apoptosis of myoepithelial and luminal epithelial cells. Our findings implicate spontaneous apoptosis in both the etiology and progression of comedo-DCIS. It is possible that spontaneous apoptosis facilitates elimination of cells thus permitting expansion and malignant transformation of cancer cells that are resistant to spontaneous apoptosis.

Rad6B is a positive regulator of beta-catenin stabilization

Mutations in beta-catenin or other Wnt pathway components that cause beta-catenin accumulation occur rarely in breast cancer. However, there is some evidence of beta-catenin protein accumulation in a subset of breast tumors. We have recently shown that Rad6B, an ubiquitin-conjugating enzyme, is a transcriptional target of beta-catenin/TCF. Here, we show that forced Rad6B overexpression in MCF10A breast cells induces beta-catenin accumulation, which despite being ubiquitinated is stable and transcriptionally active. A similar relationship between Rad6B, beta-catenin ubiquitination, and transcriptional activity was found in WS-15 and MDA-MB-231 breast cancer cells, and mouse mammary tumor virus-Wnt-1 mammary tumor-derived cells, implicating Rad6B in physiologic regulation of beta-catenin stability and activity. Ubiquitinated beta-catenin was detectable in chromatin immunoprecipitations performed with beta-catenin antibody in MDA-MB-231 but not MCF10A cells. Rad6B silencing caused suppression of beta-catenin monoubiquitination and polyubiquitination, and transcriptional activity. These effects were accompanied by a reduction in intracellular beta-catenin but with minimal effects on cell membrane-associated beta-catenin. Measurement of beta-catenin protein stability by cycloheximide treatment showed that Rad6B silencing specifically decreases the stability of high molecular beta-catenin with minimal effect upon the 90-kDa nascent form. In vitro ubiquitination assays confirmed that Rad6B mediates beta-catenin polyubiquitination, and ubiquitin chain extensions involve lysine 63 residues that are insensitive to 26S proteasome. These findings, combined with our previous data that Rad6B is a transcriptional target of beta-catenin, reveal a positive regulatory feedback loop between Rad6B and beta-catenin and a novel mechanism of beta-catenin stabilization/activation in breast cancer cells.

Inhibition of breast tumor growth and angiogenesis by a medicinal herb: Ocimum gratissimum

Ocimum sp. is a traditionally used medicinal herb, which shows anti-oxidant, anti-carcinogenic, radio-protective and free radical scavenging properties. So far no detailed studies have been reported on its effects on human cancers. Thus, we analyzed its effects on human breast cancer utilizing in vitro and in vivo methodologies. Aqueous extracts were prepared from the mature leaves of Ocimum gratissimum (OG) cultivated devoid of pesticides. Tumor progression and angiogenesis related processes like chemotaxis, proliferation, apoptosis, 3D growth and morphogenesis, angiogenesis and tumor growth were studied in the presence or absence of the extract, and in some experiments a comparison was made with purified commercially available eugenol, apigenin and ursolic acid. Aqueous OG leaf extract inhibits proliferation, migration, anchorage independent growth, 3D growth and morphogenesis and induction of COX-2 protein in breast cancer cells. A comparative analysis with eugenol, apigenin and ursolic acid showed that the inhibitory effects on chemotaxis and 3D morphogenesis of breast cancer cells were specific to OG extract. In addition, OG extracts reduced tumor size and neoangiogenesis in a MCF10 DCIS.com xenograft model of human DCIS. This is the first detailed report showing that OG leaf extract may be of value as a breast cancer preventive and therapeutic agent and might be considered as additional additive in the arsenal of components aimed at combating breast cancer progression and metastasis.

Surfactant-polymer nanoparticles overcome P-glycoprotein-mediated drug efflux

Nanoparticles enhance the therapeutic efficacy of an encapsulated drug by increasing and sustaining the delivery of the drug inside the cell. We have previously demonstrated that Aerosol OT (AOT)-alginate nanoparticles, a novel formulation developed recently in our laboratory, significantly enhance the therapeutic efficacy of encapsulated drugs like doxorubicin in drug-sensitive tumor cells. The purpose of this study is to evaluate the drug delivery potential of AOT-alginate nanoparticles in drug-resistant cells overexpressing the drug efflux transporter, P-glycoprotein (P-gp). AOT-alginate nanoparticles were formulated using an emulsion-cross-linking process. Rhodamine 123 and doxorubicin were used as model P-gp substrates. Cytotoxicity of nanoparticle-encapsulated doxorubicin and kinetics of nanoparticle-mediated cellular drug delivery were evaluated in both drug-sensitive and -resistant cell lines. AOT-alginate nanoparticles enhanced the cytotoxicity of doxorubicin significantly in drug-resistant cells. The enhancement in cytotoxicity with nanoparticles was sustained over a period of 10 days. Uptake studies with rhodamine-loaded nanoparticles indicated that nanoparticles significantly increased the level of drug accumulation in resistant cells at nanoparticle doses higher than 200 microg/mL. Blank nanoparticles also improved rhodamine accumulation in drug-resistant cells in a dose-dependent manner. Nanoparticle-mediated enhancement in rhodamine accumulation was not because of membrane permeabilization. Fluorescence microscopy studies demonstrated that nanoparticle-encapsulated doxorubicin was predominantly localized in the perinuclear vesicles and to a lesser extent in the nucleus, whereas free doxorubicin accumulated mainly in peripheral endocytic vesicles. Inhibition of P-gp-mediated rhodamine efflux with AOT-alginate nanoparticles was confirmed in primary brain microvessel endothelial cells. In conclusion, an AOT-alginate nanoparticle system enhanced the cellular delivery and therapeutic efficacy of P-gp substrates in P-gp-overexpressing cells.

Direct involvement of breast tumor fibroblasts in the modulation of tamoxifen sensitivity

Using contact-dependent three-dimensional coculture systems and serum-free conditions, we compared the ability of estrogen receptor (ER)-alpha(+) tamoxifen-sensitive premalignant (EIII8) or tumorigenic (MCF-7), ER-alpha(+) tamoxifen-resistant (EIII8-TAM(R)) or ER-alpha(-) MDA-MB-231 breast cancer cells to interact and undergo epithelial morphogenesis on association with breast tumor-derived fibroblasts. Although all breast cancer cell lines interacted with tumor fibroblasts, EIII8 and its intrinsically tamoxifen-resistant counterpart EIII8-TAM(R) cells were most receptive and responded with dramatic, albeit, aberrant epithelial morphogenesis. EIII8 cells underwent epithelial morphogenesis when cocultured with fibroblasts from ER-alpha(-)/PgR(-) or ER-alpha(+)/PgR(+) breast tumors; however, EIII8 cells cocultured with ER-alpha(-)/PgR(-) tumor-derived fibroblasts exhibited decreased tamoxifen sensitivity compared with cells cocultured with ER-alpha(+)/PgR(+) tumor fibroblasts. Fibroblast-induced tamoxifen resistance was accompanied by mitogen-activated protein kinase and Akt hyperactivation, reduced sensitivity to U0126 or LY294002, and ER-alpha hyperphosphorylation in the activation function-1 domain. The intrinsic tamoxifen resistance of EIII8-Tam(R) cells correlated with constitutive ER-alpha hyperphosphorylation that was unaffected by the tumor fibroblasts. Our results suggest that tumor fibroblast-induced tamoxifen resistance of EIII8 cells is not mediated by epidermal growth factor receptor or insulin-like growth factor (IGF)-1R axes because no correlation was found between expression levels of IGF-1, IGF-2, phosphorylated IGF-1R, or epidermal growth factor receptor, and tamoxifen sensitivity of EIII8 fibroblast cultures.

Essential role of T-cell factor/beta-catenin in regulation of Rad6B: a potential mechanism for Rad6B overexpression in breast cancer cells

We have previously shown that the postreplication DNA repair gene Rad6B plays a critical role in the maintenance of genomic integrity of human breast cells. Whereas normal breast cells express low levels of Rad6B, increases in Rad6B expression occur in hyperplasia with overexpression in breast carcinomas. Here, we show that the human Rad6B gene is a transcriptional target of T-cell factor (TCF)-4/beta-catenin/p300. Rad6B promoter activity is subject to negative regulation in normal human MCF10A breast cells whereas it is constitutively active in metastatic MDA-MB-231 breast cancer cells. Derepression and activation of Rad6B promoter in MCF10A cells require coexpression of beta-catenin and p300. Using electrophoresis mobility shift assay, Western blot analysis of electrophoresis mobility shift assay, UV cross-linking, and chromatin immunoprecipitation assay, we show that Rad6B transcriptional repression in MCF10A cells is due to paucity of transcriptionally active beta-catenin assembled on the TCF binding sequence in the Rad6B promoter rather than to a deficit/decreased affinity of TCF-4 for the TCF binding element in Rad6B promoter. Three-dimensional epithelial acini generated in vitro from MCF10A cells cotransfected with beta-catenin and p300 showed beta-catenin expression on the membrane, cytoplasm, and/or nuclei with concomitant Rad6 overexpression, whereas control acini showed beta-catenin on the membranes and negligible Rad6 expression. Immunohistochemical analysis of 12 breast carcinomas showed an approximately 80% correlation between Rad6 and beta-catenin expression, and combined nuclear and cytoplasmic staining of beta-catenin and Rad6 was detected in 25% of the breast carcinomas. In vivo implantation of MCF10A-Rad6B cells produced hyperplastic lesions. These data reveal a potentially important role for transcriptionally active beta-catenin in the regulation of Rad6B gene expression, and link aberrant beta-catenin signaling with transcriptional deregulation of Rad6B and breast cancer development.

Alterations in galectin-3 expression and distribution correlate with breast cancer progression: functional analysis of galectin-3 in breast epithelial-endothelial interactions

To define the role of galectin-3 in breast cancer progression, we have used a novel three-dimensional co-culture system that recapitulates in vivo reciprocal functional breast epithelial-endothelial cell-cell and cell-matrix interactions, and examined the expression of galectin-3 mRNA and protein in human breast tumors and xenografts. Galectin-3 is required for the stabilization of epithelial-endothelial interaction networks because immunoneutralization with galectin-3 antibodies abolishes the interactions in a dose-dependent manner. Co-culture of epithelial cells with endothelial cells results in increase in levels of secreted galectin-3 and presence of proteolytically processed form of galectin-3 in the conditioned media. In contrast, intracellular galectin-3 predominantly exists in the intact form. This difference in sensitivity to proteolytic processing of secreted versus intracellular galectin-3 probably arises from differences in accessibility of protease-sensitive sites, levels, and/or type of activated protease(s), and may be indicative of different functional roles for intact and processed galectin-3. To determine whether the proteolytically cleaved galectin-3 retains its ability to bind to endothelial cells, binding assays were performed with the full-length and matrix metallopeoteinase-2-cleaved recombinant galectin-3. Although a dose-dependent increase in binding to human umbilical vein endothelial cells was observed with both full-length and cleaved galectin-3, proteolytically cleaved galectin-3 displayed approximately 20-fold higher affinity for human umbilical vein endothelial cells as compared to the full-length protein. Examination of galectin-3 expression in breast tumors and xenografts revealed elevated levels of galectin-3 mRNA and protein in the luminal epithelial cells of normal and benign ducts, down-regulation in early grades of ductal carcinoma in situ (DCIS), and re-expression in peripheral tumor cells as DCIS lesions progressed to comedo-DCIS and invasive carcinomas. These data suggest that galectin-3 expression is associated with specific morphological precursor subtypes of breast cancer and undergoes a transitional shift in expression from luminal to peripheral cells as tumors progressed to comedo-DCIS or invasive carcinomas. Such a localized expression of galectin-3 in cancer cells proximal to the stroma could lead to increased invasive potential by inducing novel or better interactions with the stromal counterparts.

RAD6B overexpression confers chemoresistance: RAD6 expression during cell cycle and its redistribution to chromatin during DNA damage-induced response

The HR6A and HR6B genes, homologs of the yeast RAD6 gene, encode ubiquitin conjugating enzymes that are required for postreplication repair (PRR) of DNA and damage-induced mutagenesis. We show here that consistent with its role as a PRR protein, HR6 protein (referred as RAD6) expression is cell cycle regulated, with maximal levels expressed in late S/G2 phases of the cell cycle. Exposure of MCF10A cells to adriamycin (ADR) causes enhancement in the levels of RAD6B mRNA and protein. Inclusion of actinomycin D abolishes both basal and ADR-induced RAD6B transcription indicating that ADR-induced effects on RAD6B transcription result from an increase in transcriptional activity rather than from regulation of RAD6B mRNA stability. The increase in RAD6 protein expression observed in ADR-treated cells is dependent upon transcription and de novo protein synthesis, as addition of actinomycin D and cycloheximide eliminated the induction effects. Using in vivo crosslinking experiments, we demonstrate that only a small proportion of RAD6 is associated with chromatin in untreated MCF10A cells. However, treatment with ADR or cisplatin is accompanied by a significant increase and redistribution of RAD6 to DNA, and RAD6, RAD18, PCNA, phosphohistone H3, as well as p53 proteins are all found in the DNA fractions. These findings suggest that although RAD6 protein is present in the nucleus, its recruitment to the chromatin appears to be modulated by DNA damage. Whereas MCF10A cells engineered to overexpress ectopic RAD6B are significantly more resistant to ADR and cisplatin as compared to empty vector-transfected cells, MCF10A cells stably transfected with antisense RAD6B display hypersensitivity to these damage-inducing drugs. Analysis of PRR capacities in cisplatin-treated MCF10A cells stably transfected with empty vector, RAD6B or antisense RAD6B showed that whereas RAD6B-overexpressing and vector control MCF10A cells possessed the ability to convert newly synthesized DNA to higher molecular weight species, MCF10A cells depleted of RAD6B are PRR-compromised. Although no human diseases have been linked to mutations in the PRR pathway genes, these data suggest that RAD6 may play an essential role in DNA damage tolerance and recovery via modulation of PRR, and that imbalances in the levels of RAD6 could lead to changes in drug sensitivity and damage-induced mutagenesis.

Supramolecular complex formation between Rad6 and proteins of the p53 pathway during DNA damage-induced response

The HR6A and -B genes, homologues of the yeast Rad6 gene, encode ubiquitin-conjugating enzymes that are required for postreplication repair of DNA and damage-induced mutagenesis. Using surface plasmon resonance, we show here that HR6 protein (referred as Rad6) physically interacts with p53. Analysis of proteins coimmunoprecipitated with Rad6 antibody from metabolically labeled normal MCF10A human breast epithelial cells not only confirmed Rad6-p53 interactions in vivo but also demonstrated for the first time that exposure of MCF10A cells to cisplatin or adriamycin (ADR) induces recruitment of p14ARF into Rad6-p53 complexes. Further analysis of ADR-induced p53 response showed that stable Rad6-p53-p14ARF complex formation is associated with a parallel increase and decrease in monoubiquitinated and polyubiquitinated p53, respectively, and arrest in G(2)/M phase of the cell cycle. Interestingly, the ADR-induced suppression of p53 polyubiquitination correlated with a corresponding decline in intact Hdm2 protein levels. Treatment of MCF10A cells with MG132, a 26S proteasome inhibitor, effectively stabilized monoubiquitinated p53 and rescued ADR-induced downregulation of Hdm2. These data suggest that ADR-induced degradation of Hdm2 occurs via the ubiquitin-proteasome pathway. Rad6 is present in both the cytoplasmic and nuclear compartments of normal MCF10A cells, although in response to DNA damage it is predominantly found in the nucleus colocalizing with ubiquitinated p53, whereas Hdm2 is undetectable. Consistent with in vivo data, results from in vitro ubiquitination assays show that Rad6 mediates addition of one (mono-) to two (multimono-) ubiquitin molecules on p53 and that inclusion of Mdm2 is essential for its polyubiquitination. The data presented in the present study suggest that Rad6-p53-p14ARF complex formation and p53 ubiquitin modification are important damage-induced responses that perhaps determine the fidelity of DNA postreplication repair.

Host microenvironment in breast cancer development: extracellular matrix-stromal cell contribution to neoplastic phenotype of epithelial cells in the breast

Epithelial-mesenchymal interactions play an important role both in normal mammary gland development and during neoplastic transformation. Perturbations in the production, deposition and degradation of the extracellular matrix occurring during neoplastic transformation and progression have been implicated to arise from alterations in the stromal response. These changes in the stroma exhibit a dominant regulatory role, via microenvironmental epigenetic effectors, to contribute to the development of the tumorigenic epithelial phenotype. The role of stromally derived microenvironmental epigenetic effectors in modulating epithelial growth, hormonal response, morphogenesis and epithelial plasticity is discussed.

Rad6 overexpression induces multinucleation, centrosome amplification, abnormal mitosis, aneuploidy, and transformation

We have isolated by differential RNA display a cDNA that is up-regulated in metastatic mammary tumor lines. This cDNA corresponds to HR6B, the yeast homologue of Rad6, a ubiquitin-conjugating enzyme, and a key player in postreplication repair and induced mutagenesis in the yeast. We show that Rad6 protein expressed in metastatic tumor lines is wild type and functional, because it is able to catalyze the transfer of ubiquitin to histone H2b and is predominantly localized in the nucleus as compared with cytoplasmic localization in normal or nonmetastatic mammary cells. This pattern of Rad6 protein expression/localization is not restricted to breast cancer cell lines, because human breast carcinomas display similar patterns of Rad6 up-regulation and nuclear localization suggesting that deregulation in expression of Rad6 may be an important step in transformation to malignant phenotype. Constitutive overexpression of exogenous human HR6B cDNA into normal-behaving MCF10A human breast epithelial cells induced cell-cell fusion that resulted in generation of multinucleated cells, centrosome amplification, multipolar mitotic spindles, aneuploidy, and ability for anchorage-independent growth. Double immunofluorescence labeling experiments demonstrated the colocalization of Rad6 protein with gamma-tubulin on centrosomes. This physical association of Rad6 with centrosomes is maintained throughout the interphase and mitotic phases of the cell cycle. The Rad6 protein exhibits notable alterations in distribution during interphase and mitotic stages of the cell cycle that are compatible with its function as a transcription factor. These findings suggest that Rad6 is an important ubiquitin-conjugating enzyme that may play a significant role in the maintenance of genomic integrity of mammalian cells and that an imbalance in the levels and activity of Rad6 could lead to chromosomal instability and transformation in vitro.